1. If the numbers are arranged horizontally, the first row of the Excel worksheet starts from cell A 1 until there are numbers in column Z.. To calculate the arithmetic average of numbers in every three consecutive cells, you can enter the formula =AVERAGE(A 1:C 1) in cell A2, hold down the cross in the lower right corner of cell A2 and drag it to the X column on the right. The arithmetic mean of A 1+B 1+C 1 is obtained in A2, and the arithmetic mean of B 1+C 1 is obtained in B2. ...

2. If the numbers are arranged vertically, there are numbers in the first column of the Excel worksheet from the cell starting with A 1 to the A30 line. For the arithmetic average of numbers in every three consecutive cells, you can enter the formula =AVERAGE(A 1:A3) in the cell of B 1, hold down the cross in the lower right corner of B 1 and drag it to the 30th line to the right. ...

I hope it helps.